The present invention relates to a heating or preheating furnace for so-called lining cylinders made of refractory material, particularly arranged to act as molds for the lost-wax casting of precious, semiprecious or base metals or titanium or alloys thereof to obtain small items, dental prostheses or jewelry and costume-jewelry items in general by precision casting.
As is known, for producing dental prostheses use is made of molds constituted by cylinders made of refractory material whose function is not merely that of containing but also adaptating (expanding) in order to make up for contraction of the cooling metal.
In order to ensure uniform expansion during baking or firing, the mold must be uniformly heat treated to prevent regions with a high thermal gradient from being formed which would give rise to differentiated or otherwise non-uniform expansion in the mold. The mold or lining must be treated at temperatures between 800.degree. and 1000.degree. C. within a suitable muffle furnace. It is important that said heating is uniform, otherwise the refractory material is subjected to changes in its behavior in terms of location and intensity of the expansions produced by temperature increase.
A drawback is the fact that conventional muffle furnaces designed to treat lining cylinders have an internal chamber delimited by walls, not all of which are provided with heating means. Accordingly, heat is radiated unevenly inside the chamber and this means that the refractory material of the lining cylinders does not undergo uniform heat treatment in all directions.
Furthermore, by arranging a plurality of cylinders inside the furnace, the cylinders near the walls provided with heating means form shadow zones for those located behind them, thereby producing a consequent uneven heat distribution, so that a lining cylinder can have regions exposed to different temperature levels.